Estimate the wavelength of a 30 keV electron.
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Now, v = fλ, so λ = v/f, but this is where I run into a problem, as I'm not told what speed the electron is travelling at - do I assume speed of light, or is there some other way?

Hi Pyroadept!

If you can convert the eV into J, and if you know the mass of an electron, then you can calculate the speed from the energy.